Generally, isostatic refers to a technique using a fluid, either a gas or a liquid, to apply equal pressure in all directions to a sample, usually carried in a container. Sometimes isostatic is specific to gas and hydrostatic to liquid; however, herein isostatic refers to both gas and liquid, i.e. fluid materials. Isostatic pressing may be accomplished at elevated temperatures, hot isostatic pressing, or lower temperatures, cold isostatic pressing. In cold isostatic pressing the container is made of a flexible material such as rubber; in hot isostatic pressing the container is made of a deformable material such as metal, plastic or glass, which deforms at the elevated temperature. Cold isostatic pressing techniques are used in pressing samples, such as metal powder or powder compacts, into desired forms using very high pressures. The technique is not restricted to metals: it is used for other materials also; e.g. ceramics. The parts so formed are referred to in the industry as "preforms" or "compacts." After pressing, the compacts are sintered to achieve a density of approximately 94%. This technique is generally less expensive than machining or forging for many metals. Typically the pressure are delivered by mechanical presses such as hydromechanical presses whose performance is measured in hundreds of tons or by other presses which generate pressures of 50-60,000 psi. Recently isostatic presses have created much interest.
Cold isostatic techniques use powerful and expensive plumbing systems which may include complex valve and conduit networks, to increase and decrease the pressure on the sample in a pressure vessel. The isostatic medium is usually specially prepared and much energy is consumed in applying the required high pressures. For higher densities, sintering or hot isostatic pressing is used.
Hot isostatic pressing generally increases density but requires even more equipment. In hot isostatic pressing the sample is heated to softening temperature in an oven or other heating device while the container is squeezed by the fluid. The apparatus includes, in addition to a pressure vessel and compressor or pump, a furnace, vacuum pump, controls and ancillary equipment. Compressors that produce greater than 60,000 psi for cold isostatic pressing are difficult to obtain and are expensive. The capital investment required for mechanical and isostatic presses is substantial: in fact, in the case of most companies powdered metal parts are not made in-house; their production is contracted to others who are specially concerned with powder technology. Isostatic pressing is not limited to the forming of parts: it may also be used to "heal" pores and cracks containing castings and sintered bodies.